The present invention relates to solar collectors, particularly of the type in which an arcuate mirror focuses incident sunlight on a tubular absorber.
It is well known that a solar collector may be constructed by providing the combination of an arcuate mirror aimed toward the sun by a tracking device and an absorber upon which the mirror is focused; for example, a long mirror of parabolic cross-section together with a blackened smaller diameter water pipe. This is found to be relatively inefficient; losses are caused by the hot black-body absorber radiating thermal energy at a rate proportionate to its hot outer surface temperature, and conducting heat to the air passing over it. The conduction losses may be lessened greatly by providing a glass vacuum tube about the absorber pipe.
Even more refined versions of such a system utilize an enveloping vacuum tube having a silvered surface, except for "windows" which admit the light focused upon the absorber tube. The absorber pipe may be provided with a selective surface coating by which its absorptance of sunlight is an order of magnitude greater than its emittance of thermal radiation. These refinements are extraordinarily expensive and may have a short lifetime due to the extreme temperature.